Move StylePropertySet::createImmutable() to ImmutableStylePropertySet.
<http://webkit.org/b/114584>

Reviewed by Anders Carlsson.

StylePropertySet::createImmutable() -> ImmutableStylePropertySet::create()
Also, immutableCopyIfNeeded() now returns a PassRefPtr<ImmutableStylePropertySet>.

* css/CSSParser.cpp:
(WebCore::CSSParser::createStylePropertySet):
* css/StylePropertySet.cpp:
(WebCore::ImmutableStylePropertySet::create):
(WebCore::StylePropertySet::immutableCopyIfNeeded):
* css/StylePropertySet.h:
(StylePropertySet):
(ImmutableStylePropertySet):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148398 268f45cc-cd09-0410-ab3c-d52691b4dbfc